Nobody in this VN is emotionally mature enough period.
Fixed it for you
I don't agree with you that Layla is "too jealous", she's really not, she just has massive self-confidence issues, she always thinks she's "not enough" or "lesser than (her sister or other girls)", that's where her anxiousness comes from... but that's not the same as jealousy.
The ones that have shown proper jealousy so far have been Alexis & Nikki... however that's not to say that the others would be cool with sharing.
So yeah, I don't think there's gonna be a lot of poly options... The only "viable" throuple option so far is Layla/Elena, frankly because they seem to already have the hots for each other anyways and might end up hooking up if you're not on their paths.
Alexis might be open to a threesome, but probably not a throuple... Nikki no fuckin way to either. Val's made it clear she's not interested. I'd say Iris would be totally up for an Erryn/Iris throuple, but we know THAT one isn't gonna happen, unless the whole John thing is another red herring, and that'd be lame at this point. Erryn's not even up for sex yet, so anything more than that is unlikely. Curiously enough, the only other character I feel would be up for a throuple is Cara, 'cause she seems to have a really kinky side that might make her open to some more "extravagant" ideas.